As proposed already by way of Japanese Patent L-O-P Publn. No. 178708/1985, Japanese Patent Applns. Nos. 220550/1984, 236828/1984, 236829/1984, 242336/1984 and 95906/1986, it had previously been found by the present applicant that cycloolefin random copolymers obtained by copolymerization of ethylene and cycloolefins such as tetracyclodecene are synthetic resins which are excellent in transparency and also are well balanced among such properties as heat resistance, heat aging resistance, chemical resistance, solvent resistance, dielectric properties and mechanical strength and which exhibit excellent performance in the field of optical materials such as optical memory disk and optical fiber.
Further, the present applicant has found that random copolymers of ethylene and aromatic group-containing norbornenes have also such excellent characteristics as mentioned above.
The aromatic group-containing norbornenes used in preparing such random copolymers as mentioned above may be prepared by Diels-Alder reaction of cyclopentadienes with corresponding olefins. The aromatic group-containing norbornenes are obtained as isomer mixtures containing an endo-form (I-A) and an exo-form (I-B), however, because the cis addition proceeds predominantly, the endo-form (I-A) is mainly formed and very little of the exo-form (I-B) is formed. The isomer mixtures of aromatic group-containing norbornenes obtained by Diels-Alder reaction usually contain the endo-form in an amount of at least 85 mol % or more than 90 mol % in most cases.
The present inventors prosecuted extensive research with the view of improving further heat resistance and mechanical strength of such cycloolefin random copolymers as obtained by copolymerization of ethylene and aromatic group-containing norbornenes, whereupon it has been found that cycloolefin random copolymers obtained by copolymerization of isomer mixtures of aromatic group-containing norbornenes having a relatively large exo-form (I-B) content with ethylene are improved markedly in heat resistance and mechanical strength in comparison with those obtained by copolymerization of isomer mixtures of aromatic group-containing norbornenes having a relative small exo-form (I-B) content with ethylene.